The present invention relates to self-tapping fasteners, and more particularly to removable, self-tapping threaded pipe plugs that will provide a sealed plug with respect to fluids and liquids carried in the pipe, the pressure tight plug being attained preferably without thread sealing compounds.
National standards for pipe plug threads have been developed and are widely used, these are well known in the art and are referred to as National Pipe Thread standards--Fuel and oil, viz.,--NPTF standards. The standard tapered NPTF pipe plug has been developed for use in engagement with a pre-tapped pipe or workpiece. The pipe is prepared by first drilling a straight hole, which is then taper reamed to provide a tapered hole. Finally the tapered reamed hole is tapped with a NPTF tapered tap to provide an internally threaded, tapered aperture, the internal thread being formed to a prescribed, predetermined configuration. With the standard NPTF pipe plug, the roots of both the exterior and interior threads are truncated slightly more than the crests so that the roots have wider flats than the crests, which is designed to produce metal-to-metal contact and deformation occurring at the crests and roots coincident with threaded engagement, prior to attaining thread flank contact. The deformation of the crests of the internal and external threads provides a sealing action at both the major and minor diameters, in theory preventing spiral leakage through the threads. In practice, however, due to the capillary action of the fluid or gas carried in the pipe, a potential spiral leakage path exists along the helix of the screw thread, if deformation does not completely seal the spiral path.
One disadvantage of the standard NPTF pipe plug is that during installation of the NPTF plug into the pre-tapped hole cross-threading may occur, which results in an unsealed spiral path along the threads. The cross-threads may not be detected and thereby causes fluid leakage under pressure at the plug seal during service. When the cross-threading is detected, the joint often must be scraped or reworked, both of which are costly and time consuming.